Since thin microstrip antennas are inherently narrow band, it is important to have a simple means for tuning the antenna over a wide range. Based on the cavity-model theory, one can expect that the antenna could be tuned like a cavity. Some workers have demonstrated experimentally the tuning with a shorting post in the patch. It is desirable to have a theory which can predict the exact post location for a given operating frequency. To this end, our multipost theory is extended so that a simple algorithm can be applied to a wide frequency range, covering the resonant frequencies of several modes. Using this extended theory, the operating frequencies for many post locations are computed and found to agree with the measured results within a fraction of one percent. In fact, it is believed that the theory can be improved even further. It is also shown that CP can also be obtained by placing a post in a square patch. Again, the theoretical and experimental results are in almost perfect agreement.